EXPERIMENTS OF ENSEMBLE FORECAST OF TYPHOON TRACK USING BDA PERTURBING METHOD

A new method, BDA perturbing, is used in ensemble forecasting of typhoon track. This method is based on the Bogus Data Assimilation scheme. It perturbs the initial position and intensity of typhoons and gets a series of bogus vortex. Then each bogus vortex is used in data assimilation to obtain initial conditions. Ensemble forecast members are constructed by conducting simulation with these initial conditions. Some cases of typhoon are chosen to test the validity of this new method and the results show that: using the BDA perturbing method to perturb initial position and intensity of typhoon for track forecast can improve accuracy, compared with the direct use of the BDA assimilation scheme. And it is concluded that a perturbing amplitude of intensity of 5 hPa is probably more appropriate than 10 hPa if the BDA perturbing method is used in combination with initial position perturbation.     

EXPERIMENTAL STUDY ON A DYNAMIC ASYMMETRICAL TYPHOON INITIALIZATION SCHEME BASED ON 4D-VAR

Axisymmetric bogus vortexes at sea level are usually used in the traditional bogus data assimilation (BDA) scheme. In the traditional scheme, the vortex could not accurately describe the specific characteristics of a typhoon, and the evolving real typhoon is forced to unreasonably adapt to this changeless vortex. For this reason, an asymmetrical typhoon bogus method with information blended from the analysis and the observation is put forward in this paper, in which the impact of the Subtropical High is also taken into consideration. With the fifth-generation Penn State/NCAR Mesoscale Model (MM5) and its adjoint model, a four-dimensional variational data assimilation (4D-Var) technique is employed to build a dynamic asymmetrical BDA scheme to assimilate different asymmetrical bogus vortexes at different time. The track and intensity of six summer typhoons much influenced by the Subtropical High are simulated and the results are compared. It is shown that the improvement in track simulation in the new scheme is more significant than that in the traditional scheme. Moreover, the periods for which the track cannot be simulated well by the traditional scheme can be improved with the new scheme. The results also reveal that although the simulated typhoon intensity in the new scheme is generally weaker than that in the traditional scheme, this trend enables the new scheme to simulate, in the later period, closer-to-observation intensity than the traditional scheme. However, despite the fact that the observed intensity has been largely weakened, the simulated intensity at later periods of the BDA schemes is still very intensive, resulting in overly development of the typhoon during the simulation. The limitation to the simulation effect of the BDA scheme due to this condition needs to be further studied.     

APPLICATION OF BDA SCHEME IN TYPHOON TRACK PREDICTION

The MM5, which is the PSU/NCAR mesoscale nonhydrostatic limited-area model, and its adjoining modeling system are used in this paper. Taking T106 analysis data as background field the authors generate an optimal initial condition of a typhoon by using two bogus data assimilation schemes, and conduct some numerical simulating experiments. The results of No.9608 typhoon (Gloria) show that the optimal initial field have some dramatic improvements, such as inaccurate position of typhoon center, weaker typhoon circulation and incomplete inner structure of the typhoon, which are caused by shortage of data over the sea. Some improvements have been made in the track forecast. Through several comparing experiments, the initialization optimized by BDA scheme is found to be more reasonable than GFDL scheme and its typhoon track forecast is better.     

A New Typhoon Bogus Data Assimilation and its Sampling Method: A Case Study

In this study,the authors introduce a new bogus data assimilation method based on the dimension-reduced projection 4-DVar,which can resolve the cost function directly in low-dimensional space.The authors also try a new method to improve the quality of samples,which are the base of dimension-reduced space projection bogus data assimilation (DRP-BDA).By running a number of numerical weather models with different model parameterization combinations on the typhoon Sinlaku,the authors obtained two groups of samples with different spreads and similarities.After DRP-BDA,the results show that,compared with the control runs,the simulated typhoon center pressure can be deepened by more than 20 hPa to 30 hPa and that the intensity can last as long as 60 hours.The mean track error is improved after DRP-BDA,and the structure of the typhoon is also improved.The wind near the typhoon center is enhanced dramatically,while the warm core is moderate.     

The Effects of Assimilating Satellite Brightness Temperature and Bogus Data on the Simulation of Typhoon Kalmaegi (2008)

Observational and bogus satellite data are directly assimilated into the Weather Research and Forecasting (WRF) model in simulations of Typhoon Kalmaegi (2008). The data assimilation is performed using the Radiative Transfer for TIROS-N Operational Vertical Sounder (RTTOV) model and the three-dimensional variational data assimilation (3DVAR) technique, with satellite observations taken from the National Oceanic and Atmospheric Administration-16 (NOAA-16) Advanced TIROS Vertical Sounder (ATOVS) system composed of the High-resolution Infrared Radiation Sounder (HIRS), the Advanced Microwave Sounding Unit-A (AMSU-A), and the Advanced Microwave Sounding Unit-B (AMSU-B). Data assimilation experiments are initialized at three different times. Improvements in the numerical simulation of the typhoon are discussed in the context of wind, temperature, pressure, and geopotential fields. The results indicate that assimilation of satellite data can improve both the representation of the initial conditions and the subsequent simulation of the typhoon. Different satellite data have different impacts on the typhoon track. In these simulations, data from AMSU-A play a greater role in improving the simulation of the typhoon than data from AMSU-B or HIRS. Assimilation of satellite data significantly affects the simulation of the subtropical high and the steering of the typhoon by the environmental flow. The subtropical high is enhanced and extends westward in the data assimilation experiments. The background flow therefore steers the typhoon more westward, improving the simulated typhoon track. Although direct assimilation of satellite brightness temperature improves the simulated environmental conditions, it does not significantly improve the simulated intensity of the typhoon. By contrast, initializing the typhoon simulation using bogus data in tandem with satellite data improves not only the environmental conditions but also the simulated inner-core structure of the typhoon. Assimilation of both types of data therefore improves the simulation of both the typhoon track and the typhoon intensity. The results of these experiments offer new insight into improving numerical simulations of typhoons.     

EXPERIMENTAL STUDIES WITH AN OPERATIONAL NUMERICAL MODEL FOR PREDICTION OF TYPHOON TRACKS OVER THE SOUTH CHINA SEA REGION

Experiments for short range forecasting of typhoon tracks over the South China Sea Region were carried out using limited-area numerical model recently developed by the authors.Due to the shortage of sounding data over the sea, typhoon circulations are usually obtained incompletely from objective analysis, or with significant deviation of the centre from reality.Therefore a set of schemes for typhoon initialization are proposed here to construct a bogus typhoon with the circulation being consistent with the physical processes included in the limited-area model.Based on the schemes bogus data over the model grids are created and analysed together with conventional observational data.Then the normal model initialization scheme is issued to further modulate the model typhoon with the numeriCal modeel.Some experiments of sensitivity in various aspects are conducted for further improvement of typhoon track predictions.The experimental results for a number of typhoon landing in southern China in 1993 and 1994 shows that the limited-area model is capable of predicting typhoon tracks in the southern China region.     

THE EFFECTS OF BOGUS TYPHOON AND OBSERVED OCEANIC DATA ON THE ABILITY OF T_(213)L_(31) TO PREDICT TC TRACK

The abilities of typhoon (TC) track prediction by a medium-range forecast model T213L31 at National Meteorological Center are analyzed and its ability to improve its TC forecasts is discussed. The results show that about 57% of the TCs could be predicted by T213L31 but the initial position errors are large.The 43% area without the prediction of TC tracks is concentrated between 13°N and 20°N and east of 120°E and lack of conventional observation data is the main reason for the absence of TC prediction in this area.The adding of bogus TC could improve the ability of TC track prediction when there is no TC vortex in the analysis field, but could only have positive effects on the short-range TC track prediction when there is TC vortex in the T213L31 analysis field.     

四维变分同化方法在台风初始化及其数值模拟中的应用--对9608号(Herb)台风的研究结果

The MM5 and its four dimensional variational data assimilation (4D-Var) system are used in this paper. Based on the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data, the authors generate an optimal initial condition for a typhoon by using the bogus data assimilation (BDA) scheme. BDA is able to recover many of the structural features of typhoons including a warm-core vertex, the correct center position, and the strong circulation.As a result of BDA using a bogus surface low, dramatic improvement is achieved in the 72 h prediction of typhoon Herb. Through several cases, the initialization by BDA effectively generates the harmonious inner structure of the typhoon, but which is lacking in the original analysis field. Therefore the intensity forecast is improved greatly. Some improvements are made in the track forecast, but more work still needs to be done.     

INVESTIGATION ON EFFECTS OF INITIAL SCHEMES FOR BINARY TYPHOONS ROKE AND SONCA IN 2011

Based on the Tropical Region Atmospheric Modeling System for South China Sea (TRAMS), Typhoon Roke (1115) and Sonca (1116) in 2011 which have large forecast errors in numerical operation prediction, have been selected for research focusing on the initial scheme and its influence on forecast. The purpose is to find a clue for model improvement and enhance the performance of the typhoon model. Several initialization schemes have been designed and the corresponding experiments have been done for Typhoon Roke and Sonca. The results show that the forecast error of both typhoons’ track and intensity are less using the initial scheme of relocation and bogus just for the weak Typhoon Sonca, compared with using the scheme for both typhoons. By analysis the influence of the scheme on weak typhoon vortex circulation may be the reason that leads to the improvement. All weak typhoons in 2011 to 2012 are selected for tests. It comes to the conclusion that the initial scheme of relocation and bogus can reduce the error of track and intensity forecast. Besides, the height of cloud top in typhoon vortex constructed by bogus is too high according to weak typhoon. It is feasible to develop a bogus which is suitable for weak typhoon.     

BOUNDARY LAYER STRUCTURE IN TYPHOON SAOMAI (2006): UNDERSTANDING THE EFFECTS OF EXCHANGE COEFFICIENT

Recent studies have shown that surface fluxes and exchange coefficients are particularly important to models attempting to simulate the evolution and maintenance of hurricanes or typhoons.By using an advanced research version of the Weather Research and Forecasting(ARW)modeling system,this work aims to study the impact of modified exchange coefficient on the intensity and structures of typhoon Saomai(2006)over the western North Pacific.Numerical experiments with the modified and unmodified exchange coefficients are used to investigate the intensity and structure of the storm,especially the structures of the boundary layer within the storm.Results show that,compared to the unmodified experiment,the simulated typhoon in the modified experiment has a bigger deepening rate after 30-h and is the same as the observation in the last 12-h.The roughness is leveled off when wind speed is greater than 30 m/s.The momentum exchange coefficient(CD)and enthalpy exchange coefficient(CK)are leveled off too,and CD is decreased more than CK when wind speed is greater than 30 m/s.More sensible heat flux and less latent heat flux are produced.In the lower level,the modified experiment has slightly stronger outflow,stronger vertical gradient of equivalent potential temperature and substantially higher maximum tangential winds than the unmodified experiment has.The modified experiment generates larger wind speed and water vapor tendencies and transports more air of high equivalent potential temperature to the eyewall in the boundary layer.It induces more and strong convection in the eyewall,thereby leading to a stronger storm.     

轴对称模型台风的改进方案及其对0425号台风数值模拟的效果

西北太平洋台风切向风廓线存在明显的差异,目前MM5中尺度数值模式NCAR-AFWA的模型台风（bogus）方案中仅使用了最大风速和最大风速半径两个参数,有时不能准确地描述台风的外围结构。通过引入十级风圈平均半径和七级风圈平均半径对NCAR-AFWA的bogus切向风廓线进行改进,然后对0425号台风“洛坦”的路径和强度进行模拟,结果发现：在台风bogus切向风廓线中同时引入十级风圈平均半径和七级风圈平均半径对台风路径和强度模拟均有改进,同时对台风大风范围的模拟也有改进。     

非对称台风风场的动力初始化应用研究

在对比NCAR-AFWA三维Bogus(虚拟)风场及风压场方案的基础上,虚拟台风模型中的水平风场采用考虑台风移速、移向、摩擦等客观因素的非对称方案,垂直风场采用考虑高空反气旋环流的风廓线方案,然后利用Nudging动力初始化技术形成新的模式初始场.最后对"云娜"台风进行数值模拟,结果表明:通过合理引入客观因子及观测事实对台风Bogus模型进行构建,并利用同化方法使各个物理量相互协调,可以使初始场与实况更加接近,从而在一定程度上提高台风路径及强度的预报水平,并对降水分布的预报效果也有所改善.     

大涡模拟试验中台风梯度风平衡分析

尽管经典台风强度理论是基于梯度风平衡模型,但是已有的飞机观测资料和数值模拟研究发现,边界层中和眼墙附近存在非梯度风平衡气流,前人数值模拟发现,眼墙附近最大的超梯度风可以达到切向风速的16.7%。大涡模拟可以模拟出滚涡和龙卷尺度涡旋等小尺度系统,这些小尺度系统对梯度风平衡可能产生影响,使用中尺度天气预报模式结合大涡模拟（WRF-LES）对模拟台风内核区域方位角平均的梯度风平衡进行了分析,三个大涡模拟试验的水平分辨率分别为333 m、111 m和37 m,结果表明非梯度风平衡气流并未因为分辨率提高而显著增强或产生结构差异,最大超梯度风出现在边界层顶处最大切向风半径的内侧,达到梯度风速的10.8%~16.1%。进一步分析发现,不同台风中心定位方法会影响非梯度风平衡气流,最小气压方差法和最大切向风法可以得到与前人模拟一致的结构,而气压权重法得到的低层超梯度风中心远离眼墙、靠近台风中心,位涡权重法得到的低层超梯度风的径向范围向内扩大,最小气压方差法和最大切向风法更适合用于梯度风平衡研究中的台风中心定位。     

多时刻台风Bogus资料同化对台风"天鹅"的影响

基于GRAPES(Global/Regional Assimilation and Predition System)三维变分同化系统,设计在6小时内分别应用三小时一次和逐小时的台风定位和定强信息对台风进行多次BDA(Bogus Data Assimilation)同化,在多次BDA中只同化近似程度高的涡旋风分量而近似...     

台风Chanchu（0601）强度日变化特征分析

利用非静力中尺度模式WRF模拟了台风Chanchu（0601）,模式再现了台风Chanchu的路径、强度及结构。利用模式资料分析了台风Chanchu发展增强过程中其流出层和流入层风速的日变化特征、造成该日变化特征的机制及其对台风强度的影响。分析表明:台风Chanchu流出层和流入层的风速均存在显著的日变化特征,表现在低层径向入流和高层径向出流在夜间至清晨明显增强,在白天增加缓慢;切向风变化趋势同径向风类似,位相较径向风落后约6 h。通过对比夜间和白天云顶温度（CTT）和垂直速度频率（CFADS）的分布,发现夜间对流较白天更加活跃,这与夜间云顶冷却所导致的静力稳定度降低有关。利用切向风倾向方程进行收支分析,结果显示太阳辐射日变化通过调节对流日变化,引起高低层径向气流的日变化,进而造成切向风速的日变化,从而影响台风强度,在一定程度上揭示了日变化对台风强度变化的指示意义。      

A Bogus Typhoon Scheme and Its Application to a Movable Nested Mesh Model

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台风"云娜"(2004)近海加强的数值模拟研究

本文利用非静力平衡的中尺度模式MM5(V3)对2004年14号台风RANANIM("云娜")在登陆前近海加强及登陆初期的过程进行了54 h模拟,并加入人造台风优化初始场。结果表明:MM5能比较好地模拟出台风近海及登陆初期的移动路径及台风中心气压的变化。利用数值模拟结果,讨论了RANANIM(2004)台风在近海加强过程中的环流、动力和热力结构特征。发现在台风RANANIM近海加强的过程中对应有高空200 hPa净辐散场的存在,台风中心气压随净辐散值的增大而降低,反之亦然。净辐散值的减小对台风中心气压的快速升高有着预示的作用;垂直环流在强度和空间分布上发生了明显的变化。台风加强后,台风中心上方出现了明显的补偿性下沉气流;对流层低层存在明显的冷暖空气堆积,且当冷暖空气强度相当时有利于台风的增强;台风中心上方的正涡度柱逐渐发展为东西不对称分布,垂直方向上伸展高度保持不变或升高,而且在量值上也要更大。     

多层套细网格台风预报模式

本文提出一个多层套细网格台风路径数值预报模式,粗网格的预报区域包括两半球的低纬地区。粗细网格的匹配采用单方向衔接。初始台风环流是由台风中心海平面气压与最大风速所确定的一个模型环流。1984年台风季节实时预报试验取得了良好的结果。      

基于BDA扰动法的台风路径集合预报试验研究

提出一种新的方法（BDA扰动方法）对台风路径进行集合预报。BDA扰动方法是基于BDA同化方案。通过扰动台风的中心位置和台风强度得到一系列模型台风，将之分别同化到同一环境场中得到一系列扰动初始场，从而对台风进行集合预报的方法。本文选取四个台风个例进行数值试验，将直接用BDA方案同化初始场的模拟结果与采用BDA扰动方法模拟的结果进行比较，发现：利用BDA扰动方法对台风路径进行集合预报能进一步提高路径预报精度；同时发现：对于BDA扰动方法，若与初始定位扰动相结合，5hPa的台风强度扰动幅度可能比10hPa的更适宜。